How many times have I heard someone say, "I could write that...romance, mystery, fantasy novel, they're easy?" Guess what? They're not easy. The person who writes in that genre does it because he/she likes that genre.
And don't think you can write a bestseller because you think the one you just read isn't any good. The writer who wrote that book did the best she/he can. And believed in it every moment.
The most important thing is to never try to write in a genre or style of book you can't read. If you don't read horror novels, don't try to write one. You'll be doing it for the wrong reasons...to be rich and/or famous. And it won't work. If you're writing a book for those reasons it will show. It'll be lousy. And it'll never get published.